Aluminium is often produced by means of an electrolysis process using one or more aluminium production electrolytic cells. Such electrolytic cells typically comprise a bath for containing bath contents comprising fluoride containing minerals on top of molten aluminium. The bath contents are in contact with cathode electrode blocks, and anode electrode blocks. Aluminium oxide is supplied on regular intervals to the bath via openings at several positions along the center of the cell and between rows of anodes.
The electrolytic reaction taking place in the electrolytic cells generates a hot effluent gas that comprises gaseous components that are undesired in the atmosphere, including hydrogen fluoride, sulphur dioxide, and the like. The process also generates fine dust. The undesired gaseous components and the dust must be disposed of in an environmentally conscientious manner; hence, the raw gas is cleaned in a gas cleaning unit, to preferably remove any undesired components as efficiently as possible. Dust and gaseous components such as hydrogen fluoride may be returned to the aluminium production cells, where they may be of benefit for the production process.
A typical gas cleaning unit may comprise a dry scrubber and a dust filter, e.g. a fabric filter that may be of the bag filter type. A consideration of such systems is that energy-consuming fans are often necessary in flue gas treatment systems to actively draw the raw gas through the gas cleaning unit. Such is the case since the gas cleaning unit may introduce a flow resistance in the flue gas treatment system.
Another consideration of current gas cleaning units is that the same may constitute a significant investment and may require significant installation space. Still another consideration is that current flue gas treatment systems may require cooled flue gas. This requirement may be dictated by, e.g., the temperature sensitivity of any downstream equipment, or by the temperature dependency of the cleaning efficiency of the gas cleaning unit. In many installations, the flue gas is cooled upstream of the gas cleaning unit by diluting it with atmospheric air of ambient temperature.
WO 2008/113496 suggests, as an alternative solution to dilution with ambient temperature air, the use of a heat exchanger resistant to scaling, i.e. the formation of deposits of dust and fouling on its inner surfaces, which makes it particularly suitable for cooling uncleaned gas.
Finally, gas cleaning units should preferably offer high reliability, or, alternatively, back-up systems should be provided, since it may be very expensive and/or troublesome to stop and re-start the electrolytic aluminum production process that depend on effective operation of associated gas cleaning units.